Why word "nigger" is perojative and disdainful? I am not american and i am certainly not ar racist and i would like to know this. And also - why gansters and rappers call each other like that but do not allow white people to do that?

Because by calling someone that, you are effectively stating that they are your slave. Obviously it is different to call someone "fellow slave" than "my slave"

Many words that were offensive a century ago are now OK. Some words that were OK then are now offensive.
In the very early 20th century, "idiot", "moron", and "imbecile" were perfectly acceptable. Now they are insults with meanings similar to the original. In the 1960's and 1970's, racial epithets were considered very offensive. Today they are still offensive depending on the context. Words pertaining to mental and physical disabilities are now also offensive, but were originally OK.

You got it confused. Rappers call each other niggas, not niggers, which is similar, but it means friend, or person. Nigger is the word slave owners used to call black people back 150+ years ago, that's why it's considered offensive. Personally, I just think that the word became "forbidden" because people made it forbidden. To use the word nigga without fear, you have to be either black, be in an area without black people, or the black people you are with are cool about it (The word is very informal, hence rap music). I'm an 18 year old white dude, and my generation says it all the time. We (white ppl) usually just ask "hey, you don't mind if I say nigga", and 99% of the time, people 25 years old and younger don't care, but you have to watch out, cuz it's really really sensitive. BUT NEVER SAY NIGGER! I'll even kick your ass if you say it cuz I have a lot of black friends.